Dunn, Alexander James Aiken (2018) The controlled hydro/solvothermal synthesis of ceria-based materials. PhD thesis, University of Warwick.

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Abstract

This thesis reports on the one-pot hydro/solvothermal synthesis of a range of mixed metal cerium oxides with potential catalysis applications. The produced materials were characterised using a wide range of techniques to understand the synthesis-dependent variations in crystallite size, morphology, surface absorbed species and redox properties.

The hydrothermal synthesis of bismuth-substituted cerium dioxide is reported. The choice of starting reagents in the hydrothermal reaction was found to affect the crystallite sizes of the resulting Ce0.5Bi0.5O2-δ samples. The Ce0.5Bi0.5O2-δ samples with the lower crystallite size and hence larger surface areas were derived from the use of Ce(NH4)2(NO3)6 as the cerium source. In situ X-ray diffraction and X-ray absorption experiments indicate that the cubic structure of Ce0.5Bi0.5O2-δ is formed via formation of poorly ordered CeO2 followed by the reduction of bismuth and/or to the crystallisation of α-Bi2O3 from the bismuth starting reagents prior to its inclusion into CeO2.

The solvothermal reaction of CeCl3 with LnCl3 (Ln = La, Pr, Gd and Tb) in polyethylene glycol at 240 °C yields a range of previously unreported Ce1-xLnx(OH)2Cl samples with a unusual morphology. Barring lanthanum, up to approximately 50% lanthanide-substitution was achieved. Contrarily, the series of Ce1-xLnx(OH)CO3 materials prepared via the use of nitrate precursors achieved only 20% lanthanide-substitution. The thermal decomposition of the lanthanide-substituted cerium precursors produces materials that adopt the fluorite type unit cell and retain their unique crystal morphology. X-ray absorption near-edge structure spectra recorded at the Pr and Tb K-edges indicates the existence of both in two oxidation states, Tb3+/Tb4+ and Pr3+/Pr4+. The results deduced from XANES spectra of the mixed oxides agree with the calculated lattice parameters refined from the X-ray diffraction patterns. The incorporation of lanthanide cations improves the reducibility of ceria as measured by H2-temperature programmed reduction.

Three methods using alkoxide starting reagents have successfully been able to synthesise the ordered k-Ce2Zr2O8 phase. Powder X-ray diffraction shows that the cationic ordering can be achieved at temperatures as low as 1000 °C without the use of reducing gases. A study of both the local environment and structure of the zirconium substituted ceria samples reveals that the solvothermal method of synthesis produces materials that contain a larger quantity of Ce3+.

Item Type:	Thesis or Dissertation (PhD)
Subjects:	Q Science > QD Chemistry
Library of Congress Subject Headings (LCSH):	High temperature chemistry, Cerium oxides, Catalysis, Crystallization
Official Date:	September 2018
Dates:	Date Event September 2018 UNSPECIFIED
Institution:	University of Warwick
Theses Department:	Department of Chemistry
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Walton, Richard I.
Format of File:	pdf
Extent:	162 leaves : illustrations, charts, photographs
Language:	eng

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